In the art of assembling integrated circuit die into packages that can be handled and placed into printed circuit boards by the customer, it has long been the practice to use a lead frame, which is a frame of leads that encircles the die to which the bonding pads on the die are electrically connected. In addition, integrated circuit packaging has traditionally been divided into two groups, ceramic packages which may be hermetically sealed by the flowing of glass between a ceramic base and cap, which surround the die, and plastic packages in which the die is first physically and then electrically bonded to the lead frame, which is then suspended in a mold and the plastic package material is injection molded around the die. The plastic package is not hermetic and does not protect the die as well as the ceramic package does.
In ceramic package assembly, the lead frames have an opening or hole in the center of the lead frame for receiving the die and through which wire bond connections to the bonding pads on the surface of the die are passed. The die and the proximal portions of the leads closest to the die are encased in a ceramic package, leaving the distal portions of the leads, which radiate out away from the die area and form the distal package leads or pins exposed to be inserted into holes on a printed circuit board for connection with the rest of the circuit. Usually the distal ends of the leads are connected to each other and held in fixed position relative to one another by means of an encircling rectangular frame or boundary which is severed after encasement, but prior to insertion in a printed circuit board.
In the assembly of ceramic integrated circuit packages, the integrated circuit die is first mounted on a physical support such as a shallow cavity in the bottom of a ceramic package base. To keep the die in a fixed position relative to the proximal ends of the leads of the lead frame, the die is secured into place in the cavity of the ceramic package base and the lead frame is fixed into the ceramic package base by melting a thin layer of glass on the top surface of the base. The lead frame is positioned such that the hole in the center of the leads frames or surrounds the die. Usually the assembly of base, lead frame and die is designed so that the bonding pads of the die are at about the same level or slightly below the level of the proximal ends of the leads. Next, the leads of the lead frame are connected to the bonding pads on the integrated circuit die by wire bonding.
Often, besides the bonding pads on the top side of the integrated circuit die, the bottom side of the die, also called the back side of the substrate, must be electrically connected to be properly grounded or back-biased so that the chip will function properly. It is efficient to electrically connect both the top surface bonding pad and the substrate backside together in some fashion so that only one of the leads and one of the pins need be dedicated to this electrical connection.
In ceramic packages, the hollow or shallow cavity in the ceramic base portion is filled with a gold frit material or some other conductive material and a lead must somehow be connected to this surface which is in electrical contact with the back side of the chip. The most direct way of making this connection is to "down bond" a wire directly from a lead to the gold frit surface. However, if aluminum wire is used, the gold/aluminum bond tends to be unreliable. Also, the bonding of the die to the gold frit surface tends to form a region of gold/silicon alloy around the chip which must be avoided if a good wire bond is to be made. In many cases, the region of gold/silicon alloy will be extensive enought to make it physically difficult to avoid it and make the downbond wire connection. Of course, once the downbond connection is made from the bonding pad or the lead, another wire bond connection is made to the lead or to the bonding pad, repsectively, depending on what element remains electrically unconnected to the common lead.
One common way of making this backside connection in a ceramic package is to use a bonding island. The die cavity or shallow compartment in the ceramic base is designed to be somewhat larger than the actual die size permitting room for a tiny piece of conductive silicon to also be bonded on the gold frit or other conductive material, next to the chip as a separate "island". Then the bonding pad on the surface of the chip is wire bonded to the island, which is in electrical connection to the backside of the chip via the gold frit material, the island in turn being wire bonded to the appropriate lead of the lead frame. Or the lead may have two wire bond connections, one to the bonding island and one to the bonding pad. One advantage of using a bonding island is that the island does not have to avoid the gold-silicon alloy region as do aluminum downbond wires. Often more than one bonding pad on the top surface of the chip needs to be grounded or have the same signal as the backside and the package assembly will two or more bonding islands. For an example of the use of a bonding island, see U.S. Pat. No. 4,558,436.
However, down bond islands have the disadvantages of having to separately fabricate and mount each island, which add extra expense and steps to the assembly process. One of the major disadvantages of conventional ceramic packaging is that the assemblies just described do not lend themselves to processing strips of lead frames in a continuous process. Rather, each package must be assembled individually in a "one-up" fashion which is very time consuming.
It is generally accepted by those skilled in the art that teachings for one form of assembly, namely plastic packages, cannot be transferred to the ceramic packaging art. Plastic packaging art will now be discussed for the purpose of contrast.
In the plastic packaging art, die bond flags, which are integral parts of the lead frame, are used to support the integrated circuit chip prior to wire bonding the surrounding leads to the bonding pads on the semiconductor chip. This separate support is necessary since the packaging material is not provided in discrete pieces as in ceramic packages, but is instead in pellet form, prior to injection molding around the wire bonded die/lead frame assembly. In the typical case, the die bond flag is held in its position relative to the ends of the leads of the lead frame by separate bars or supports whose sole purpose is to support and hold the die bond flag in proper position. After the die and proximal portions of the leads are encapsulated in plastic, the supports are severed from the boundary around the lead frame and the supports no longer function since the die bond flag is now held fixed by the plastic. For an example of a typical lead frame, see U.S. Pat. No. 4,523,371 to Wakashima.
Philofsky, et al. in U.S. Pat. No. 4,527,185 have developed the lead frame assembly 10 shown in FIG. 1 for plastic packages. Lead frame assembly 10 is composed of lead frame 12 having a plurality of leads 14 with distal ends 16 oriented around the outside periphery of the frame 12 and connected to rectangular holding frame 18 and proximal ends 20 oriented toward the center of lead frame 12. Philofsky, et al.'s lead frame 12 does not have a central die bond flag, as discussed above, however, there are two die support and power supply leads 22 which have distal ends 16, but instead of proximal ends 20, terminate instead in shunting capacitor holding brackets 24. The two shunting capacitor holding brackets 24 are not physically or electrically connected to one another, but are bridged by shunting capacitor 26 which forms the physical support for integrated circuit die 28. Die 28 must have some electrical connection to power supply leads 22, such as wire bonds 30, and of course, one of these connections is the ground. While the Philofsky, et al. design helps minimize the number of leads by using the two power supply leads as two support leads and makes the shunting capcaitor much more compact and brings it inside the package, there is no provision for or recognition of the need for a backside or substrate ground or bias connection, which would be particularly difficult since the integrated circuit die is mounted on the side of shunt capacitor 24.
In U.S. Pat. No. 4,514,750, Adams teaches a lead frame that removes the need for "flying leads", which are the shorter sides of outermost rectangle 18 holding the digital ends 16 of the leads 18 in FIG. 1. Adams' lead frame assembly 32, depicted in FIG. 2 has all of the vertically oriented pieces of metal as active metal leads 34 which possess proximal ends 36 and distal ends 38 held in a fixed position relative to each other by outside rectangle 40. Outside rectangle 40 has two short sides, the bulk of which will be used as leads, although the longer sides will be severed from the distal ends 38 of leads 34. Thus by Adams' design, very little metal is wasted. Another unusual feature of the Adams lead frame assembly 32 is that the lead frame provides a die bond flag 42 for a ceramic, hermetic chip package.
The Adams lead frame assembly 32 does have a die bond flag 42 in the center of the lead frame assembly 32 toward which the proximal ends 36 of the leads 34 point. Like many common lead frame designs, such as U.S. Pat. No. 4,523,371 to Wakashima, Adams' die bond flag is physically supported from the outside rectangle 40 by two separate, dedicated flag supports 44, which are severed at nibs 46 after package assembly via niches in the package body. However, a special grounded lead 48 is disclosed by Adams, that is connected to one of the supports 44 via grounding conection 50. Thus, integrated circuit die 52 of Adams, may have its backside or bottom substrate grounded via physical and electrical connection to die bond flag 42, and a top die surface bond pad bonded via wire bond 54 to the proximal end 56 of ground lead 48 which is joined to the die bond flag by connection 50. Thus, while Adams solves the problem of grounding or back biasing the bottom of the die 52 unlike Philofsky, et al., One skilled in the art does not find from Adams a teaching of minimizing the number of leads, since essentially three structures, the two supports 44 and the ground lead 48 serve the functions of support and electrical connection, respectively.
It would be a desirable advance in the art if a lead frame for ceramic integrated circuit chip packages were discovered which could combine the advantages of Philofsky, et al. and Adams without the disadvantages left by each.